Engineering Mechanics Unit

Bangalore, India

Engineering Mechanics Unit

Bangalore, India

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Chao L.,Nanyang Technological University | Kwak S.K.,Ulsan National Institute of Science and Technology | Ansumali S.,Engineering Mechanics Unit
International Journal of Modern Physics C | Year: 2014

We propose a modified direct simulation Monte Carlo (DSMC) method, which extends the validity of DSMC from rarefied to dense system of hard spheres (HSs). To assess this adapted method, transport properties of hard-sphere (HS) systems have been predicted both at dense states as well as dilute, and we observed the excellent accuracy over existing DSMC-based algorithms including the Enskog theory. The present approach provides an intuitive and systematic way to accelerate molecular dynamics (MD) via mesoscale approach. © 2014 World Scientific Publishing Company.

Raja R.V.,National Institute of Technology Tiruchirappalli | Subramanian G.,Engineering Mechanics Unit | Koch D.L.,Cornell University
Journal of Fluid Mechanics | Year: 2010

The behaviour of an isolated nearly spherical drop in an ambient linear flow is examined analytically at small but finite Reynolds numbers, and thereby the first effects of inertia on the bulk stress in a dilute emulsion of neutrally buoyant drops are calculated. The Reynolds numbers, Re = ya 2p/μ and Re =ya2p/μ are the relevant dimensionless measures of inertia in the continuous and disperse(drop) phases, respectively. Here, a is the drop radius, is the shear rate, is the common density and and are, respectively, the viscosities of the drop and the suspending fluid. The assumption of nearly spherical drops implies the dominance of surface tension, and the analysis therefore corresponds to the limit of the capillary number(Ca) based on the viscosity of the suspending fluid being small but finite; in other words, Ca ≤ 1, where Ca =μay/T, T being the coefficient of interfacial tension. The bulk stress is determined to O(øRe) via two approaches. The first one is the familiar direct approach based on determining the force density associated with the disturbance velocity field on the surface of the drop; the latter is determined to O(Re) from a regular perturbation analysis. The second approach is based on a novel reciprocal theorem formulation and allows the calculation, to O(øRe), of the drop stresslet, and hence the emulsion bulk stress, with knowledge of only the leading-order Stokes fields. The first approach is used to determine the bulk stress for linear flows without vortex stretching, while the reciprocal theorem approach allows one to generalize this result to any linear flow. For the case of simple shear flow, the inertial contributions to the bulk stress lead to normal stress differences(N 1, N2) at O(øRe), where (≤1) is the volume fraction of the disperse phase. Inertia leads to negative and positive contributions, respectively, to N1 and N2 at O(øRe). The signs of the inertial contributions to the normal stress differences may be related to the O(ReCa) tilting of the drop towards the velocity gradient direction. These signs are, however, opposite to that of the normal stress differences in the creeping flow limit. The latter are O(Ca) and result from an O(Ca2) deformation of the drop acting to tilt it towards the flow axis. As a result, even a modest amount of inertia has a significant effect on the rheology of a dilute emulsion. In particular, both normal stress differences reverse sign at critical Reynolds numbers(Re c) of O(Ca) in the limit Ca≤1. This criterion for the reversal in the signs of N1 and N2 is more conveniently expressed in terms of a critical Ohnesorge number(Oh) based on the viscosity of the suspending fluid, where Oh = μ/(paT)1/2. The critical Ohnesorge number for a sign reversal in N1 is found to be lower than that for N2, and the precise numerical value is a function of Λ In uniaxial extensional flow, the Trouton viscosity remains unaltered at O(øRe), the first effects of inertia now being restricted to O(Re 3/2). The analytical results for simple shear flow compare favourably with the recent numerical simulations of Li & Sarkar (J. Rheol., vol. 49, 2005, p. 1377). Copyright © Cambridge University Press 2010.

Koch D.L.,Cornell University | Subramanian G.,Engineering Mechanics Unit
Annual Review of Fluid Mechanics | Year: 2011

Experimental observations indicate that, at sufficiently high cell densities, swimming bacteria exhibit coordinated motions on length scales (10 to 100 μ) that are large compared with the size of an individual cell but too small to yield significant gravitational or inertial effects. We discuss simulations of hydrodynamically interacting self-propelled particles as well as stability analyses and numerical solutions of averaged equations of motion for low Reynolds number swimmers. It has been found that spontaneous motions can arise in such systems from the coupling between the stresses the bacteria induce in the fluid as they swim and the rotation of the bacteria due to the resulting fluid velocity disturbances. © 2011 by Annual Reviews. All rights reserved.

Subudhi S.,National Institute of Technology Calicut | Sreenivas K.R.,Engineering Mechanics Unit | Arakeri J.H.,Indian Institute of Science
Journal of Fluids Engineering, Transactions of the ASME | Year: 2012

This paper deals with the study of a submerged jet for the suction of unwanted fluid. This submerged jet is caused by the fluid coming out from a source. The presence of a sink in front of this source facilitates the suction of the fluid depending upon the source and sink flow rates, the axial and lateral separations of the source and sink, and the angle between the axes of the source and sink. The main purpose is the determination of the sink flow rate for 100 % removal of the source fluid as a function of these parameters. The experiments have been carried using a source nozzle 6 mm in diameter and two sizes for the sink pipe diameter: 10 mm and 20 mm. The main diagnostics used are flow visualization using dye and particle image velocimetry (PIV). The dependence of the required suction flow rate to obtain 100 % effectiveness on the suction tube diameter and angle is relatively weak compared to the lateral separation. © 2012 American Society of Mechanical Engineers.

Somasekhar M.,National Aerospace Laboratories, Bangalore | Vivek S.,Indian Institute of Technology Madras | Malagi K.S.,National Aerospace Laboratories, Bangalore | Ramesh V.,National Aerospace Laboratories, Bangalore | Deshpande S.M.,Engineering Mechanics Unit
Communications in Computational Physics | Year: 2012

In the present work adaptation in meshless framework is proposed. The grid adaptation or mesh adaptation is quite well developed area in case of conventional grid based solvers and is popularly known as Adaptive mesh refinement (AMR). In such cases the adaptation is done by subdividing the cells or elements into finer cells or elements. In case of mesh less methods there are no cells or elements but only a cloud of points. In this work we propose to achieve the meshless adaptation by locally refining the point density in the regions demanding higher resolution. This results into an adaptive enriched cloud of points. We call this method as Adaptive Cloud Refinement (ACR). The meshless solvers need connectivity information, which is a set of neighboring nodes. It is crucial part of meshless solvers. Obviously because of refining point density, the connectivity of nodes in such regions gets modified and hence has to be updated. An efficient connectivity update must exploit the fact that the node distribution would be largely unaffected except the region of adaptation. Hence connectivity updating needs to be done locally, only in these regions. In this paper we also present an extremely fast algorithm to update connectivity over adapted cloud called as ACU (Automatic Connectivity Update). © 2012 Global-Science Press.

Prasianakis N.,Paul Scherrer Institute | Ansumali S.,Engineering Mechanics Unit
Communications in Computational Physics | Year: 2011

The exact solution to the hierarchy of nonlinear lattice Boltzmann kinetic equations, for the stationary planar Couette flow for any Knudsen number was presented by S. Ansumali et al. [Phys. Rev. Lett., 98(2007), 124502]. In this paper, simulation results at a non-vanishing value of the Knudsen number are compared with the closed-form solutions for the higher-order moments. The order of convergence to the exact solution is also studied. The lattice Boltzmann simulations are in excellent agreement with the exact solution. © 2011 Global-Science Press.

Ansumali S.,Engineering Mechanics Unit
Communications in Computational Physics | Year: 2011

This work proposes an extension to Boltzmann BGK equation for dense gases. The present model has an H-theorem and it allows choice of the Prandtl number as an independent parameter. I show that similar to Enskog equation this equation can reproduce dynamics of dense gases. © 2011 Global-Science Press.

Alam M.,Engineering Mechanics Unit | Shukla P.,IISER Kolkata
AIP Conference Proceedings | Year: 2013

In micro-structural fluids, the homogeneous shear flow breaks into alternate regions of low and high shear rates (i.e., shear localization), respectively, when the applied shear rate exceeds a critical value and this is known as gradient banding. On the other hand, if the applied shear stress exceeds a critical value, the homogeneous flow separates into bands of different shear stresses (having the same shear rate) along the vorticity (spanwise) direction, leading to stress localization, and the resulting pattern is dubbed vorticity banding. Here we provide a brief overview of our recent work on nonlinear order-parameter theory to describe various pattern formation scenario in a sheared granular fluid, with a specific focus on the vorticity-banding phenomena. The analysis holds for any general constitutive model, but the results are presented for a kinetic-theory constitutive model that holds for rapid granular flows. Our theory predicts that the vorticity banding can occur via supercritical/subcritical pitchfork and subcritical Hopf bifurcations in dilute and dense flows, respectively, resulting in an inhomogeneous state of shear stress and pressure. © 2013 AIP Publishing LLC.

Ansari I.H.,Engineering Mechanics Unit | Alam M.,Engineering Mechanics Unit
AIP Conference Proceedings | Year: 2013

We report experimental results on pattern formation in vertically vibrated granular materials confined in a quasitwo-dimensional container. For a deep bed of mono-disperse particles, we uncovered a new transition from the bouncing bed to an f/4-wave (f is the frequency of shaking) which eventually gives birth to an f/2-undulation wave, with increasing shaking intensity. Other patterned states for mono-disperse particles and their transition-route are compared with previous experiments. The coarse-grained velocity field for each patterned state has been obtained which helped to characterize convective rolls as well as synchronous and sub-harmonic waves in this system. © 2013 AIP Publishing LLC.

Sandoval M.,University of California at San Diego | Sandoval M.,Metropolitan Autonomous University | Marath N.K.,Engineering Mechanics Unit | Subramanian G.,Engineering Mechanics Unit | And 2 more authors.
Journal of Fluid Mechanics | Year: 2014

Most classical work on the hydrodynamics of low-Reynolds-number swimming addresses deterministic locomotion in quiescent environments. Thermal fluctuations in fluids are known to lead to a Brownian loss of the swimming direction, resulting in a transition from short-time ballistic dynamics to effective long-time diffusion. As most cells or synthetic swimmers are immersed in external flows, we consider theoretically in this paper the stochastic dynamics of a model active particle (a self-propelled sphere) in a steady general linear flow. The stochasticity arises both from translational diffusion in physical space, and from a combination of rotary diffusion and so-called run-and-tumble dynamics in orientation space. The latter process characterizes the manner in which the orientation of many bacteria decorrelates during their swimming motion. In contrast to rotary diffusion, the decorrelation occurs by means of large and impulsive jumps in orientation (tumbles) governed by a Poisson process. We begin by deriving a general formulation for all components of the long-time mean square displacement tensor for a swimmer with a time-dependent swimming velocity and whose orientation decorrelates due to rotary diffusion alone. This general framework is applied to obtain the convectively enhanced mean-squared displacements of a steadily swimming particle in three canonical linear flows (extension, simple shear and solid-body rotation). We then show how to extend our results to the case where the swimmer orientation also decorrelates on account of run-and-tumble dynamics. Self-propulsion in general leads to the same long-time temporal scalings as for passive particles in linear flows but with increased coefficients. In the particular case of solid-body rotation, the effective long-time diffusion is the same as that in a quiescent fluid, and we clarify the lack of flow dependence by briefly examining the dynamics in elliptic linear flows. By comparing the new active terms with those obtained for passive particles we see that swimming can lead to an enhancement of the mean-square displacements by orders of magnitude, and could be relevant for biological organisms or synthetic swimming devices in fluctuating environmental or biological flows. © 2014 Cambridge University Press.

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